The present invention relates to an image processing device for emphasizing a specified luminance portion of an image taken by an image sensor.
Usually, in the case of taking a picture of an object having a bright objective portion (high light portion) and a dark surrounding portion (shadow portion) such as for example a road viewed toward the outlet (or the inlet) of a tunnel from the inside (or the outside) thereof for monitoring a white line marked on the road in front of a vehicle running thereon or an image of a working site where welding, laser machining or plasma machining is conducted, it is highly required to use an image sensor having a sufficiently wide dynamic range.
Referring to FIG. 27, there is shown a state of a weld joint being formed by the laser beam welding on a metal surface where the weld portion comprises a key hole, a molten pool formed by heating with a laser beam at a high temperature and a bead portion formed by solidified weld metal behind the molten pool. To determine whether the welding is well conducted, it is needed to monitor the states of the high bright keyhole portion including the molten pool and the low bright bead portion at a time on the same image. For this purpose, it is needed to use an image sensor having a sufficiently wide dynamic range.
Japanese Laid-Open Patent Publication No. 11-211565 discloses a conventional MOS type image sensor composed of a plurality of light sensor circuits, each of which represents one unit pixel and comprises, as shown in FIG. 1, a photo-diode PD for producing a sensor current proportional to the quantity of incident light Ls falling thereon, a transistor Q1 for charging and discharging a parasitic capacity C of the photodiode, a transistor Q2 for amplifying the voltage signal Vpd at a terminal of the photodiode PD and a transistor Q3 for outputting a sensor signal in accordance with a timing pulse of a readout signal Vs, wherein the sensor current produced in the photodiode is converted into a voltage signal Vpd by the transistor Q1 having a logarithmic output characteristic in a weak inverse state and having a gate voltage VG fixed at a certain level to serve as an overflow drain.
The above-mentioned image sensor can produce an output having logarithmic characteristic while a sufficient quantity of light Ls is falling on the photodiode PD. However, if the sensor circuit current decreased with a small quantity of incident light, then its output characteristic becomes non-logarithmic because of a delayed response to charging and discharging parasitic capacity C. In FIG. 3 hereof, there is shown a non-logarithmic response area WA and a logarithmic response area WB.
Japanese Laid-Open Patent Publication No. 11-21565 discloses an image sensor composed of a plurality of light-sensor circuits (FIG. 1) each representing a single pixel, which is improved so that output characteristic of each sensor circuit at a small quantity of incident light may be logarithmic in all the output range and the sensor output may be converted into a specified output characteristic using a lookup table to reduce an amount of information and facilitate post-processing such as for example multi-gradient sampling. However, the image sensor using light-sensor circuits having a logarithmic output characteristic for attaining a wide dynamic range can take an image covering a wide range from a high bright portion to a dark portion but it may suffer insufficient contrast of the image because of logarithmic compression of luminance.